Roofing question.

Aug 18, 2017 6 Replies
D

Have a flat roof (standard roofing felt) that meets up with a sloping slated one. The flat roof is angled slightly so the slate roof takes all the rain water from it.



The joint between the two is covered in lead.



Even when new, I'd had an occasional problem with a leak which seems to be from there - but can't be sure as there is plasterboard covering it in the inside. But when it does leak, water seems to run down one of the joists for the sloping roof (where you can see them, some way down from that join)



Now lead isn't going to provide a seal against driving rain, so just how should this joint be constructed?



I'd expect the felt to overlap the slate, and the lead just there to protect things?


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C

One of the problems with lead is that it doesn't work well on long runs. I believe about 2M is the maximum length. I've had a lead gutter develop a fatigue crack which i had to bitumen coat to keep waterproof.

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M

Which way round do you mean?

\ \ \_________

or __________ \ \ \

Mine is of the latter form and the slope is the other way so that the joint is a global maximum and water always wants to run off the lead.

So that each higher part overlaps the lower one (and maybe some soft mastic water repellent compound between the felt and the lead). Capilliary action has a bad habit of making water run uphill. (as does driving rain)

If it is how I imagine the lead overlaps the slate and the felt overlaps the lead. Otherwise there is always a path for driving rain to get in.

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D

The second, as water tend to flow downhill. ;-)

I do have the first arrangement at the other end of the roof near the apex of the main roof. That has never given problems.

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M

That is what I was trying to determine. Type I is quite common where there is a ground floor extension whereas type II is loft extension.

Though on mine the flat roof drains away from the apex which is more reliable in terms getting the lead to seal and avoid leaks.

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N

+1 for mastic to "glue the lead down" to the felt.
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D

How about differential expansion? Lead tends to move quite a bit.

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